The superiority of the so-called "on edge" type of cutting insert of tungsten carbide or the like is now widely recognized, with inserts of that type being secured in pockets milled in the bodies of the host milling cutters by a countersink head retaining screw passed through a countersunk hole extending between the major faces of the insert, and into a tapped hole in the cutter body.
For the type of service to which this invention is particularly directed, namely a gang slot mill for the simultaneous milling of multiple and relatively narrow slots or grooves in a workpiece, the customary retaining screw, positioned parallel to the cutter axis, and passed through a hole in the more or less radially upstanding insert, is not satisfactory because the hole for the retainer screw in an insert of the thinness required for this service does not leave sufficient cross-sectional area to resist the stresses encountered in the intended service, the thickness of the cutter body at the site of the retaining screw does not provide sufficient thread depth to anchor an insert securely after repeated adjustment, and because access to axis-parallel retaining screws in ganged slot cutters for the indexing and replacement of the cutting inserts is not feasible without disassembling the cutters from the common arbor or the like upon which they are assembled.